Chronic infections and re-infections by human cytomegalovirus (HCMV) cannot be eliminated by the host's immune system despite an extraordinary strong T cell response. Viral stealth strategies preventing the activation of immune cells and the recognition of virally infected cells are thought to be essential to escape immune eradication. Inhibiting the presentation of viral antigens by major histocompatibility complex (MHC) molecules is thought to play a key role in cytomegaloviral immune escape. However, the role of inhibiting MHC presentation for HCMV pathogenesis and persistence has not been established since infections by cytomegaloviruses are highly host-restricted and HCMV does not infect immunocompetent animals. Besides chimpanzee CMV, which is not a feasible animal model, the closest relative of HCMV is Rhesus CMV (RhCMV) which infects non-human primates. Similar to HCMV, we observed that RhCMV is capable of re-infecting seropositive animals and establishing a persistent infection. Using this new model, we will test the hypothesis that preventing expression of MHC I is essential for the establishment and maintenance of persistent infection in immunocompetent hosts. We show that RhCMV encodes inhibitors of MHC assembly that are functional and sequence homologues of the HCMV US6-family of glycoproteins US2, US3, US6 and US11. We further demonstrate RhCMV encodes additional modulator(s) of MHC I expression within a 3kb genomic fragment, Rh175-180 that encodes RhCMV-specific genes but also overlaps with the transcript of Rh181, the RhCMV homologue of HCMV US1. This novel mechanism, termed viral interference with heavy chain expression (VIHCE), acts post-transcriptionally but prior to completion of heavy chain synthesis. Thus, VIHCE precedes inhibition of MHC I assembly and transport by US6-related proteins. The goals of this proposal are i) to identify which of the gene products encoded in the Rh175-181 region are responsible for VIHCE, ii) to characterize the molecular mechanism of VIHCE, and iii) to determine the role of preventing MHC I expression, assembly or transport for escaping immune detection by CD8+ lymphocytes during establishment and maintenance of persistent infection of immunocompetent animals.